Fleece being wet is the usual reason for variation in weight at this stage; they weigh them again when they sort them, so any water weight gets taken off at that time.
If it isn't wet, which you say it isn't - and presuming you have the same number of sheep clipped in there - then the next thing that occurred to me is, is it the same time of year or has your clip been delayed by the weather?
Then things like, were there the same number of ewes who've bred lambs and hoggs who haven't; your lambing percentage similar this year to last - anything that would affect how hard the ewes have worked on things other than growing fleece. And is the average age of your flock the same or younger - although I'm not sure I think younger sheep grow more or less fleece than older; hoggs' fleeces are usually thicker, of course, as it's their first fleece and over 12 months' growth.
After that, we have to look at pay (feed) and conditions. If their minerals have been corrected over previous years, that could certainly account for it - being healthier (for this or any other reason) would leave them more resources to put into wool growth, plus sheep that have been sick have often lost some or all of their wool by the time they get clipped, so if you've had less sheep being ill, having pregnancy or rearing problems, their fleeces would be better than in previous years.
There is of course a possibility that it's reaction by the sheep to the two previous very severe winters. We haven't noticed a difference in our sheep - but the proof will be the submission weight. We're booked in for next week, I'll let you know if it's similar or much different to previous years.